Automatic merge from submit-queue (batch tested with PRs 52227, 52120)

Use COS for nodes in testing clusters by default, and bump COS.

Addresses part of issue #51487. May assist with #51961 and #50695.

CVM is being deprecated, and falls out of support on 2017/10/01. We shouldn't run test jobs on it. So start using COS for all test jobs.

The default value of `KUBE_NODE_OS_DISTRIBUTION` for clusters created for testing will now be gci. Testjobs that do not specify this value will now run on clusters using COS (aka GCI) as the node OS, instead of CVM, the previous default.

This change only affects testing; non-testing clusters already use COS by default.

In addition, bump the version of COS from `cos-stable-60-9592-84-0` to `cos-stable-60-9592-90-0`.

```release-note
NONE
```
/cc @yujuhong, @mtaufen, @fejta, @krzyzacy